1. Field of the Invention
The present invention relates to a focus servo device in an optical disc recording/reproducing apparatus, and more particularly to the focus servo device provided with a capture range based on an analysis of a lead-in range of focus servo.
2. Description of the Related Art
A focus-error detection system of the focus servo in a conventional optical disc recording/reproducing apparatus is basically designed to increase the sensitivity of the focus-error detection and thereby to extend a range of the focus-error detection. However, in this design, there are the restrictions of the capture range (which is referred to as the “CR” in this application as occasion demands) of the focus servo as described below.
Firstly, it is necessary for the focus servo to detect the deviation of the focus position caused by an external disturbance and keep the deviation of the focus position within the depth of focus by using an actuator.
Regarding to this, according to the standard of a DVD, the focus servo is defined such that the amount of surface-runout or surface-wobble of the disc is at most plus or minus (±) 0.3 mm and such that the focus position is within a range of ±0.23 μm, which is the depth of focus to allow the obtaining of a good reproduction signal. Therefore, the focus-error detection range needs to be at least −0.23 μm or less, or +0.23 μm or more; otherwise there is a possibility that the focus will be off. Generally, the focus-error detection range is about ½ of the CR in a linear region of a focus-error signal (FES) of S-curve.
Therefore, the lower limit of the CR is as follows:CR lower limitDVD≧0.23×2×2=0.92 [μm].
Moreover, such a method has been taken that the CR of the focus servo in considering a multi-layered disc is ¼ of an interlayer thickness or less, as disclosed in Japanese Patent Application Laying Open NO. Hei. 8-315370, for example. Applying this method to a DVD with Numerical Aperture (NA) 0.6, for example, since a thickness between two layers is defined to be about 55 μm in the DVD standard, the following expression applies:CR upper limitDVD≦55/4=13.75 [μm].
Taking a Blu-ray Disc (BD) for an example, which has a further increased recording capacity, since a focus residual of ±0.23 μm defined by the DVD is converted into ±0.07 μm using a ratio of the depth of focus ∝ (NA2), the CR needs to be 0.07×2×2=0.28 μm or more. Considering the multi-layered disc, since the interlayer thickness of the DVD is converted into about 17 μm using the ratio of the depth of focus ∝ (λ/NA2), the CR needs to be 17/4=4.25 μm or less. Thus, the CR range in the focus-error detection system is as follows:0.28 [μm]≦CRBD≦4.25 [μm].
In the DVD, for example, the defocus margin, the focus close position of the focus servo, or the like have never been considered because the tilt tolerance has more effect on the reproduced signal than the defocus tolerance does.
Shown in FIG. 1 is the comparison of a wavefront aberration RMS (Root Means Square) value [λ] generated by the defocus in the DVD with that of the BD. FIG. 1 teaches that the BD has a larger effect of the defocus on the aberration value than the DVD does. With respect to a wavefront aberration RMS value [λ] generated by the tilt, the DVD and the BD have the substantially same aberration value, as shown in FIG. 2.
Therefore, if the numerical aperture NA of an object lens is increased more than that of the conventional DVD and a light source wavelength λ is shorten in order to make the recording capacity as large as that of the BD, the defocus has an increased effect on the reproduction signal, so that the defocus margin becomes important in determining system margin.
In this case, as is conventionally done, if the CR of the focus servo, i.e. the focus close position, is not considered, the focus servo may close at a position deviating widely from the above-described optimum focus position. On that account, the defocus margin may decrease, which causes a possibility that the system does not function.